concentration of hydrogen ions

Question

Recall from chemistry that pH=-log([H⁺]). If the concentration of hydrogen ions in a liquid is doubled, what is the effect on the pH?

Answer 1

Logarithms are a way to express an exponential function.  We often use logs to find the value of the exponent with base number 10.

 

Suppose H⁺ = 2.  Then

 

PH = -log(2)

 

We can write this as

 

10^pH = 2

 

pH = - log 2 / log 10

pH = -0.301

 

Lets double the H⁺.

 

pH = -log(4)

 

10^pH = 4

 

pH = - log 4 / log 10

pH = -0.602

 

If we divide -0.602 by -0.301, we get a result of 2.  Therefore, the pH is halved because it went from -0.301 to -0.602.

Answer 2

Hi Ashley,

Since pH=-log([H+]), then if the concentration becomes 2[H+], you have pH=-log(2[H+]) = -{log([H+]) + log 2} = -log([H+]) - log 2

so the pH is decreased by log 2 = .301

I hope this helps.